Sajed,
The CODEC is the standard being used to digitize the analog audio signal for transmission over a digital channel. There are many available, with a variety of properties (bandwidth consumption, quality, price (in the case of proprietary ones) etc.). Not all will be supported by every endpoint.
This setting is used to configure which CODECs you wish to support for various communication paths and the preferred selection in the case of both endpoints supporting more than one.
If you have not already done so, I strongly suggest you attend either the ICCE (if you work for a Partner) or ICCS (If you work for a customer) training from Genesys Beyond as all this is covered in those classes. Making changes without fully understanding the consequences can degrade the performance of your system or, at the extreme, even cause an outage.
HTH